Filtering circuits



Aug. 2, 1932. G. H. STEVENSON FILTERING CIRCUITS Filed April 17. 1930 T, Q w 0 I INVENTOA G- H. .Srsvsnsm ATTORNEY ally not serve to reduce'the num e of P '7 Patented Aug. 2, 1932 Nita-i, r

GEORGE E. srrivnnson, or SOUNDlBE A OI-I, PHONE LABORATORIES, rnoonronarnnor new; YORK, N. 3., A CORPORATION or NEW YORK Application filed April 17,

This invention relates to selective' transmission networks and-more particularly 'to networks adapted for use-in superheterodyne receivers.

In radio receiving sets of the superheterodyne type, as is well known, the incoming radio wave is modulated with a'locally generated wave to produce a beat wave, usually of somewhat lower frequency, which is better adapted for amplification than is the inv the amplifying stages are tuned. Interference results when'signals are recelved sn'nul- 'taneously' over these two waves." Such 1nterference is particularly prevalent in the ordinary broadcasting range in which the operating frequencies of the broadcasting stations are spaced at equal frequency intervalsof about 10,000 -c. p; s. The intermediate frequency of the receiver is usually fixed, and the result is that-the interfering frequency is separated from the desired signal by a fixed frequency "interval. This latter fact is of importance in connection with the designing of expedients for removing the difficulty occasioned by the intereference', as will be pointed out more particularly'hereinafter. i

Changing the beat frequency of the local oscillator will alleviate a given case of interference. -The change, however, will gen sibleinterferences but merely to'bringin a different combination of interfering signals, generally of the same magnitude as the particular interference which has been avoided.

Sharper tuning may be resorted to as a oonnjnorrcnr, Assrsnoa 'ro BELL were- I FILTERING oIRoUrrs i 1930. semi my. 445,060.

means of reducing the strength of the interto be toosharp to admit the full band of frequencies required for faithful reproduction of signals, especially in the case of'the transmission of speech and music.

The principal objects of the present invention are first; the. suppression of the undesired beat frequency andsecond the pr0vision of band selectivity for the desired signals.

Another object of the'present invention is to permit the selection of a band of. fre quencies the width of which remains substan tIallY'COIIStELIIt as the tuning is varied. P

Another objectis to provide for ready adjustment of a selected band throughout a wide range of frequencies. Y

A further object is to provide'for substantially uniform eliiciency of transmission" throughout the received band. its l It is well known that a network comprising a series of coupled tun-ed circuits-will have the transmissioncharacteristics of a band filter when provided with properly proportioned damping means in the terminal meshes. If the transmission band is relatively narrow the resistance inherent in the tuning coils may providesuflicient' damping to produce a band characteristic in the absence of properly proportioned terminal im pedances. Itis found, however, that the band width varies with the tuning adjustment of such a system. In order to maintain a constant'band width over an extended tuning range it is necessary to vary the coupling in such a manner as to offset the tendency toward variation of the band width. I1

A selective transmission circuit which has been proposed ina copending application of H. T. Budenbom, Serial No; 374,742,' filed June 29, 1929,consists of a pair of tuned in put and output circuits coupled together by an inductively connected link circuit containing a suppression member or wave trap. The link circuit is designed'to providea suitable amount 'of coupling at each frequency so "that a substantially uniform bandwidth is secured over a wide range of tuning adjustments, and at the same-time to suppress the transmission of waves separated from the selected band by a fixed interval.

In the present invention there are employed tuned input and output circuits and a tuned coupling circuit which latter also serves as a wave trap. The three tuned circuits are directly connected either in series or in parallel, thus obviating the necessity of employing an inductively coupled link circuit. The adjustable tuning elements of the circuits are me ch anically connected together and may be operated by means of a single control. When the tuning elements are thus varied, the width of the transmission band is maintained sub stantially constant and transmission is suppressed at a frequency separated from the band by a fixed frequency interval. The tuning coils employed contain suflicient inherent resistance to provide the degree of damping necessary for maintaining substantially uniform transmission throughout the band.

The invention may be best understood from the following description with reference to the drawing, in which Fig. 1 shows one circuit embodying the invention, Fig. l-A shows amodificationof a portion of the cireuit of Fig. 1, Fig.2 is a simplified diagram showing the electrical equivalent of a portion of the. circuit of Fig. 1, Fig. 3 shows a second form of circuit embodying the invention, and Fig. f is a simplified diagram showing the electrical equivalent of a. portion of the circuit of Fig. 2.

Referring to Fig. 1, a transmission network N, embodying theinvention is disclosed with suitable connections for coupling an antenna AN to a space discharge device or vacuumtube V The network N comprises two similar branches L 0 and L VC respectively, and a dissimilar branch L 0 The three, branches are connected in parallel relation between the junction points A and A. The elements L L and L are substantiallypureinductances. and the elements C C and C2 are capacitances, also substantiallypure. The inductance L is illustrated as forming the secondary winding of. a transformer T which loosely couples the antenna AN to the network N The antenna is connected through the primary winding of transformer T to ground GND. The particular method employed for coupling an antenna or other supply circuit to the network N is not essential. to the utilization of this invention and maybe varied as desired.

The terminals B and B of network N1 are illustrated as being connected to the input .terminals of a vacuum tube. V An oscillator O is also shown connected to the input circuit ofV for supplying abeat frequency to the tube.

The oscillator O has a tuning adjustment whereby the beat frequency can be regulated.

The capacitances C C and C are all of the adjustable type and their variable parts are mechanically coupled to each other and to the tuning element of the oscillator in such manner that all may be adjusted simultaneously by turning a single handle or knob. This, adjustment serves to control the beat frequency, the tuning or location of the limits of the transmission band in the frequency scale and the frequency at which the coupling between the antenna and the tube is suppressed.

For the purposes of this specification, a resonant frequency of a network having input and output terminals is defined as any frequency at which a maximum or indefinitely large voltage is developed across the output terminals when electrical energy of the same frequency is supplied to the input terminals. Similarly, an anti-resonant frequency is a minimum voltage is developed across the output terminals or at which no voltage at all will be developed.

The network N has two resonant frequencies and one anti-resonant frequency, as defined above. The resonant frequencies determine approximately the edges of a band of free transmission. The anti-resonant frequency is the frequency at which the coupling between the meshes of the network 'is suppressed.

The equations which determine the. resonant and anti-resonant frequencies of network N in terms of the values of the inductances and capacitances will now be derived.

Fig. 2 is a simplified circuit diagram which approximately represents the network N and the effect of the reactions between the network and the antenna AN and vacuum tube V respectively. Z represents the impedance of either circuit branch L C or L C Z is the impedance of the branch L C v The electromotive force e represents the induced voltage which is impressed upon the network N by an applied wave incident upon antenna AN and effective through transformer T. X is the reactance of the condenser C The voltage V efi'ective across the reactance X is substantially that applied; to the tube V by the network l The voltage effective across the parallel branches Z and Z is denoted by V.

The electromotive force 6, in. accordance with the well known laws of electric circuits, divides between the series branch Z and the parallel combination of Z and Z in the proportion which the impedances of the respective portions bear to'the total impedance of the circuit, The impedance of the parallel combination of Z and Z is Z Z' (Z a-Z The total impedance over which e is distributed is Z +Z Z /(Z +Z The ratio of these two impedances isthe proportional part of e which is equal to V. This reladefined as any frequency at which tion may be expressed by the following equation.

' 1 2/( 1 2) I (I) V 21+ Z Z /(Z Z 6 The equation-may be simplified by dividing both the numerator and the denominator b Z and multiplying both by (Z +Z The result of this operation is p v By substituting in Eq. (3) the value of V given by Eq. (2) the voltage V derived from the network may be expressed in terms of the impressed electromotive force 6. The

result is as follows:

The circuit of Fig. 2 is resonant when the I voltage V is infinitely large whatever the value of 6 may be. By inspection of Eq. (4) it is found that resonance occurs when either Z or the combination (Z +2Z is equal to zero. The equations, which determine the resonant frequency are Expressed in words, the'resonant frequency defined by Eq. (5) is the frequency of resonance of the branch Z and thatdefined by Eq. (6) is the frequency at which the branch Z resonates with twice Z The circuit of Fig. 2 is anti-resonant whe the voltage V is zero whatever the value of 6 may be. This occurs when Z is equal to zero, as will be evident from further inspece tion of Eq. (4). The equationfor determining anti-resonance is Anti-resonance obviously occurs when the branch containing the impedance Z becomes .a short circuit due to its own resonance.

A closely approximate formula for determining the. band width of the network N Will now be derived. For convenience the following abbreviations will be used The quantities m and lo are the resonant frequencles of the nnpedances Z and Z respectively, both expressed in electrical radlans per'second. Frequencies so expressed will be referred to as angular frequencies to distinguish them from the. more usual frey quencies' expressed in cycles per second. The

"angular frequency o constitutes one limit of the transmission band. The other limiting frequency will be designated m and the band width may then be expressed as (.0 w The value of (1) is determined by resonance of Z with 2Z2 as indicated by Eq. (6). In terms 7 of thevalues of the reactance elements.

' /1 2 w /1 2 i I L1 Because the coupling between the two meshes of network N will ordinarily be small, C

' will be small in comparison with C and L will be small in comparison with L It is well known that if a quantity 00 is a small fraction the following approximation will be found to hold:

By applying this approximation to Eq. (10)- the following result is obtained: p

The value of the band width readily follow L 0J2 w w m L1 w1 1), OI

Ma 2 (91V 0 l)( k Eq. (14), shows that if the inductances are not varied and if the interval (w w is maintained constant the band width remains nearly constant, diminishing slightly as the frequency (0 increases. If the interval (w w is'not very great the diminution of the band width with increasing frequency is very small.

The limiting frequency (.0 is varied by adjusting thevariable condensers C and G simultaneously, keeping the two always equal in value. The suppression frequency m is varied by adjusting the value of C The condensers C C and C are mechani cally coupled and the blades are given such shapes that the frequencies (0 and m are at all times separated'by a predetermined and fixed frequency interval. A simple method of maintaining the fixed frequency interval is to. use condensers of the so called straight line frequency type and to have the condenser C mechanically displaced from the 7 others. The beat frequency oscillator, also pedance of network N rent is represented by L, in Fig. The vol"- mechanically coupled is simultaneously adjusted, usually to a frequency which is'separated-from (1) by another predetermined and fixed interval. The beat frequency and the suppression frequency are commonly spaced at equal frequency intervals from the center of the transmission band.

Fig. 1-;A shows a modification of the branch L G of Fig. 1. As noted above, in

order that the coupling between the meshes of network N shall'be small C will be several times as large. as C In case the required value of C is inconveniently large, a transformer T. may be employed to couple a smaller condenser G with an inductance L The inductance L may be equal to L if desired. C however may be several timesas small as and the effect of the transformer will be to magnify the capacity of C in proportion tothe ratio of the turns in the respective windings of the transformer T...

Fig. 3 discloses an arrangement alternative to that of Fig. 1. The network N is illustrated as being employed to couple the output circuit of a vacuum tube V with the input circuit of another tube V N consists of three anti-resonant circuits connected in series relationand is the converse of N of Fig. 1, which consists of three resonant cir cuits connected in parallel. A beat frequency oscillator O is coupled by means of a transformer T to the grid electrode of V and is provided with a tuning adjustment.

The networkN is adjusted by means of variable inductances L L and L which are mechanically coupled to each other and to the tuning element of the oscillator G. The network has two resonant frequencies which determine the respective limits of its transmission band and an anti-resonant frequency at which the coupling between the input and output termi ls of the network is suppressed.

The equ. as which determine the resonant and anti-resonant frequencies of network N will now be derived.

Fig. -i is a simplified circuit diagram which pproximately represents the network N and the effect of the reactions between the network and the tubes V and V respectively. The tube V acts substantially as a source of alternating current of constant amplitude because the internal impedance of the tube is generally large in comparison with the im- This constant cur age V across impedance Z is applied to the input circuit of tube V The current 1 is divided between the two parallel paths of the circuit of Fig. 4. in proportion to the admittances of the respective paths. The current in the output path will be designated L, and is determined by the following-equation:

The voltage V is determined by the following equation:

(16) V= Z 1 I Eq. (16) shows that there is a maximum value of V when Z is infinite and another when 2Z +Z is equal to zero. Theseconditions determine the-limiting frequencies of the transmission band and may be expressed as follows:

' (17) Z w (18) 2Z +Z =O Eq. (16) shows further that there is a minimum or Zero value of V when Z is infinite. This condition determines the frequency at which the coupling between the input and output terminals of the network is suppressed and may be expressed as follows:

By a process of mathematical deduction similar to that applied to the circuit of Fig. 2 it can be shown that the band width for the circuit of Fig. 4: is approximately determined by the following equation:

The band width is nearly constant when L L and L are varied, provided C and C are constant (or maintained in a fixed ratio) and the interval between (1) and 00 is kept constant. For greatest constancy of the band width, m and 00 should also be fairly close together.

The limiting frequency (0 is varied by adjusting the variable inductances L and L simultaneously, keeping the two always equal in value. The suppression frequency w is varied by adjustin the value of L The variable inductances are mechanically coupled in such manner that the frequencies m and (D are at all times separated by a predetermined and fixed frequency interval. The beat frequency oscillator also mechanically coupled, is simultaneously adjusted, usually to a frequency which is separated from m by another predetermined and fixed interval.

What is claimed is:

1. A selective network comprising input and output tuned circuits and a tuned coupling circuitconnected in parallel with said input and output circuits, the tuning elements of said circuits being mechanically connected whereby said input and output circuits are maintained in syntony and transmission therebetween is suppressed at a frequency different by a substantially constant amount from the frequency of tuning of said syntonous circuits.

2. A selective network comprising series resonant input and output circuits and a series resonant coupling circuit connected in parallel with said input and output circuits, the tuning elements of said circuits being mechanically connected. whereby said input and output circuits are maintained in syntony and transmission between said circuits is suppressed at a frequency differing by a substantially constant amount from the fre quency of tuning of the said syntonous circuits.

3. A selective circuit comprising tuned input and output branches and a tuned coupling branch each branch consisting of a fixed inductance and a variable condenser connected in series, said branches being connected in parallel and said variable condensers being mechanically connected whereby said input and output branches are maintained in syntony and transmission therebetween is suppressed by said coupling branch at a frequency different by a substantially constant amount from the frequency of tuning of said syntonous circuits.

4. A selective circuit comprising input and output tuned circuits and a tuned coupling circuit connected in parallel with said input and output circuits, the tuning elements of said circuits being mechanically connected whereby said input and output circuits are maintained in syntony and said coupling circuit is maintained in resonance at a frequency differing from the frequency of said syntony by a substantially constant amount.

5. A selective circuit comprising input and output tuned circuits" a tuned coupling circuit connected in parallel with said input and output circuits. a source of alternating currents. said source being adiustable as to freouencv. means mechanicallv connecting the tuning elements of said circuits and of s id source. whereby said input and output circuits are maintained in syntony while said coupling circuit and said source are respectively maintained in adiustment at frequencies differing from the frequenc of said syntony by substantiall constant amounts.

6. A selective network comprising input and output tuned circuits and a tuned coupling circuit of relativelv small coupling effect connected in parallel with said input and output circuits to adapt said network to transmit a band of frequencies, the tuning elements of said circuits being mechanically connected whereby said input and output cir cuits are maintained in syntony, transmission therebetween is suppressed at a frequency different by a substantial amount from the frequency of tuning of said syntonous circuits, and the transmission band is maintained of substantially constant width over a range of tuning adjustments.

7. A selective network comprising input and output tuned circuits the tuning elements of which are mechanically connected for the maintenance of syntony and a tuned coupling circuit connected in parallel with said input maintenance of syntony, a tuned coupling circuit connected in parallel with said syntonous circuits, said coupling circuit being adapted to maintain a substantially constantwidth band transmission characteristic throughout the tuning range and including a variable element mechanically coupled to the tuning elements of the syntonous circuits whereby transmission between said circuits is suppressed at a frequency differing by a substantially constant amount from the frequency of tuning of said syntonous cir cuits.

In witness whereof, I hereunto subscribe my name this 14th day of April, 1930.

' GEORGE H. STEVENSON. 

